1. Field of the Invention
The invention relates generally to data processing, and more particularly to a method for the efficient storage and retrieval of digital map data based on a common global coordinate system constructed as a tessellated geoid having polar, equatorial and temperate zones.
2. Discussion of the Prior Art
A significant problem facing the fielding of a production airborne digital map system is accommodating the variety of data formats necessary for the diversity of mission navigational requirements. The Digital Land Mass System (DLMS) data base produced by the Defense Mapping Agency (DMA) consists of Digital Terrain Elevation Data (DTED) and Digital Feature Analysis Data (DFAD) in a digital format on magnetic media. FIG. 1 generally illustrates this system where data is rendered into a pixel image by a Digital Map Computer (DMC) 10 which is installed aboard a military aircraft. The pixel image is converted into a display 11 for projection onto a CRT or reproduction on a flat panel type of display 15 Similarly, aeronautical charts are digitized into pixel data and stored on a plurality of Digital Memory Units (DMU) 12 which serve as the mass storage device for the DMC. A DMU typically uses information stored in digital form on an optical disk cartridge 13. Other data includes digitized photographs, mission data, threat/intelligence data, checklists and command files.
Storing this diverse data, managing it, and retrieving it rapidly for display is a complex task. Optimizing a storage and retrieval scheme for one data type has proved to be inefficient for a different type. Coordinate systems carried for each different data type have increased the overall complexity and cost of such systems since separate projection equations must be invoked depending upon data type.
Prior art for navigation in the airborne cockpit consists of two different methodologies. The first requires that the aviator cut up aeronautical charts into strips, which when pasted together, provide coverage for his flight plan. These strips of chart are bound in a booklet and strapped to the aviator's knee. The aviator draws his planned flight path on the charts and makes other relevant notations directly on the paper.
The second known method consists of a film strip and projection system. FIG. 2 illustrates a typical approach for producing a filmstrip data base from existing paper maps. For example, 45 inch by 58 inch paper map 14 may first be reduced by a factor of 6:1 to a microcard 20. Then sixteen microcards 20 may be mounted together in a paste-up 22, which is photographed onto the 33 mm filmstrip 24 with a further reduction of 2.5:1. The resulting map displayed on the color CRT is the same size as the paper map. The filmstrip is typically 20 meters long and contains up to 512 24 mm.times.36 mm slides. The film is photographed with about a 50% overlap so that there are no "black corners" as the map is traversed.
The shortcomings of the strip chart prior art are that it is limited in scope of coverage and flexibility if the aviator deviates from the original flight plan. The different scales cannot be nested or manipulated and overlays that are written on cannot be decluttered. Magnification of line details such as contour lines and small text font cannot be magnified nor can any other forms of manipulation associated with a digital image be performed. The various projections cannot be aligned so that parallel lines from one chart to another would connect on the filmstrip. Flight paths traverse a filmstrip too quickly if the path is perpendicular to the strip. This leads to annoying "thrashing" since the filmstrip is constantly rewinding to access the correct frame.
In both the cut and pasted strip charts and the remote map reader filmstrips, digital data cannot be accommodated. Indeed, the processing for the rendering and display of such information did not exist until the introduction of the digital map computer. For aeronautical charts, changes between map projections cannot be adjusted with this storage scheme. A Polar Stereographic (PS) projection of a chart at high latitudes is filmed and displayed as such. A Lambert Conformal Conic (LCC) projection at lower latitudes is similarly processed. The boundary between an LCC chart and a PS chart is significantly noticeable as the aircraft flies from one map sheet to the other map sheet. The discontinuities at these seams is objectionable to an aviator. Even within the same type of projection, charts cannot be made to line up exactly and the seams are quite noticeable.
The invention provides advantages and features not found in the prior art in that, as one feature of the invention, it presents a common framework not only for aeronautical charts of different scales and projections of a given gaming area, but also for other digitized pixel information such as aerial reconnaissance photographs and LandSat imagery and digital information such as Digital Terrain Elevation Data. By compensating for varying coordinate systems and projection equations, the invention allows diverse data to be stored and retrieved in a single format. This feature of the invention greatly simplifies the processing and display hardware of the Digital Map Computer. The treatment of the polar zones by the invention is a completely new and improved approach. Other digital map computers utilize a small, contiguous gaming area of square segments and do not address transpolar navigation. The discontiguous gaming area is known to some extent but no other known system is capable of reproducing the invention's long flight paths.
Another advantage of the invention is that the tessellated geoid's common coordinate system and minimal distortion overcomes the shortcomings of the prior art. By controlling the upper and lower cell dimensions of a tile (or tessellation) through the use of zones, discontiguous gaming areas do not exceed the maximum allowable distortion regardless of the distance from the origin of the gaming area. Since the data is stored in cells based on latitude and longitude and not on any particular map projection, a seamless transition from cell to cell is achieved. The display of data is limited only by the cells populated in the data base and extends easily beyond the coverage required for a particular flight plan. DTED data is readily accommodated in the tessellated structure as the distinct elevation posts are formatted in arc second measure of latitude and longitude.